The present disclosure relates to a light emitting device.
As a white light emitting device employing a light emitting diode (hereinafter also referred to as “LED”), there is, for example, a light emitting device in which a blue light emitting LED and a yellow light emitting phosphor are combined. This light emitting device emits white light by combining the blue light from the LED with the yellow light from the phosphor that is excited by the blue light. Such a light emitting device can have high emission intensity and high emission efficiency in the visible light spectrum, but might not have sufficient emission intensity in the blue-green region and the red region. For this reason, there is room for improvement in how the color of an illuminated object appears (hereinafter referred to as “color rendering properties”).
Here, the evaluation procedure for the color rendering properties of a light source is specified by JIS Z8726 where the test colors (R1 to R15) having prescribed reflectance characteristics are measured using a test light source and a reference light source, and numeric calculations of the color differences ΔEi (i is an integer 1 to 15) are performed to obtain the color rendering indexes. The upper limit of a color rendering index Ri (“i” is an integer 1 to 15) is 100. That is, the smaller the color difference between the test light source and the reference light source having the corresponding color temperature is, the higher and closer to 100 the color rendering index results. Among the color rendering indexes, the average of the values for R1 to R8 is referred to as the average color rendering index (hereinafter also referred to as Ra), and R9 to R15 are referred to as special color rendering indexes. R9 is defined as red, R10 yellow, R11 green, R12 blue, R13 the skin color of a westerner, R14 the color of leaves, and R15 the skin color of a Japanese person.
In order to enhance the color rendering properties of a light source, for example, Japanese Unexamined Patent Application Publication No. 2003-535477 proposes a light emitting device having an LED which emits blue light and two phosphors which emit green to yellow light, such as chlorosilicate phosphor and Y or Tb garnet phosphor. Moreover, in order to further enhance the color rendering properties, for example, Japanese Unexamined Patent Application Publication No. 2008-034188 proposes a light emitting device employing a phosphor which emits red light in addition to a phosphor that emits green to yellow light.